Add fork choice EF tests (#2737)

## Issue Addressed

Resolves #2545

## Proposed Changes

Adds the long-overdue EF tests for fork choice. Although we had pretty good coverage via other implementations that closely followed our approach, it is nonetheless important for us to implement these tests too.

During testing I found that we were using a hard-coded `SAFE_SLOTS_TO_UPDATE_JUSTIFIED` value rather than one from the `ChainSpec`. This caused a failure during a minimal preset test. This doesn't represent a risk to mainnet or testnets, since the hard-coded value matched the mainnet preset.

## Failing Cases

There is one failing case which is presently marked as `SkippedKnownFailure`:

```
case 4 ("new_finalized_slot_is_justified_checkpoint_ancestor") from /home/paul/development/lighthouse/testing/ef_tests/consensus-spec-tests/tests/minimal/phase0/fork_choice/on_block/pyspec_tests/new_finalized_slot_is_justified_checkpoint_ancestor failed with NotEqual:
head check failed: Got Head { slot: Slot(40), root: 0x9183dbaed4191a862bd307d476e687277fc08469fc38618699863333487703e7 } | Expected Head { slot: Slot(24), root: 0x105b49b51bf7103c182aa58860b039550a89c05a4675992e2af703bd02c84570 }
```

This failure is due to #2741. It's not a particularly high priority issue at the moment, so we fix it after merging this PR.

beacon_node/beacon_chain/tests/store_tests.rs

@@ -4,7 +4,6 @@ use beacon_chain::attestation_verification::Error as AttnError;
 use beacon_chain::builder::BeaconChainBuilder;
 use beacon_chain::test_utils::{
     test_spec, AttestationStrategy, BeaconChainHarness, BlockStrategy, DiskHarnessType,
-    HARNESS_SLOT_TIME,
 };
 use beacon_chain::{
     historical_blocks::HistoricalBlockError, migrate::MigratorConfig, BeaconChain,
@@ -19,6 +18,7 @@ use std::collections::HashMap;
 use std::collections::HashSet;
 use std::convert::TryInto;
 use std::sync::Arc;
+use std::time::Duration;
 use store::{
     iter::{BlockRootsIterator, StateRootsIterator},
     HotColdDB, LevelDB, StoreConfig,
@@ -1812,6 +1812,8 @@ fn weak_subjectivity_sync() {
     let log = test_logger();
     let temp2 = tempdir().unwrap();
     let store = get_store(&temp2);
+    let spec = test_spec::<E>();
+    let seconds_per_slot = spec.seconds_per_slot;
 
     // Initialise a new beacon chain from the finalized checkpoint
     let beacon_chain = BeaconChainBuilder::new(MinimalEthSpec)
@@ -1823,7 +1825,7 @@ fn weak_subjectivity_sync() {
         .store_migrator_config(MigratorConfig::default().blocking())
         .dummy_eth1_backend()
         .expect("should build dummy backend")
-        .testing_slot_clock(HARNESS_SLOT_TIME)
+        .testing_slot_clock(Duration::from_secs(seconds_per_slot))
         .expect("should configure testing slot clock")
         .shutdown_sender(shutdown_tx)
         .chain_config(ChainConfig::default())
@@ -2055,6 +2057,8 @@ fn revert_minority_fork_on_resume() {
     let mut spec2 = MinimalEthSpec::default_spec();
     spec2.altair_fork_epoch = Some(fork_epoch);
 
+    let seconds_per_slot = spec1.seconds_per_slot;
+
     let all_validators = (0..validator_count).collect::<Vec<usize>>();
 
     // Chain with no fork epoch configured.
@@ -2160,7 +2164,7 @@ fn revert_minority_fork_on_resume() {
             builder = builder
                 .resume_from_db()
                 .unwrap()
-                .testing_slot_clock(HARNESS_SLOT_TIME)
+                .testing_slot_clock(Duration::from_secs(seconds_per_slot))
                 .unwrap();
             builder
                 .get_slot_clock()